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APPENDIX                                     307

Mandelic acicl was originally derived from bitter almonds, and
can be obtained by the action of baryta on amygclalin, the
glucoside of bitter almonds, which breaks up into glucose and
manclelic acid. Mandelic acid contains an asymmetric carbon
atom, and is capable, therefore, of being resolved into optical
enantiomorphs (p. 262). This has been effected by fractional
crystallisation of the cinchonine salt, from a solution of which
the dextro-rotatory component first separates. Another method,
known as the biochemical method, is to cultivate certain low
organisms in a solution of a salt of the acid when one of the
components is destroyed or assimilated. Thus ordinary green
mould (penirilliuiii) assimilates and removes thekevo component,
leaving a dextro-rotatory solution. These two methods, together
with the separation of the enantiomorphous crystalline forms
described on p. 123, comprise the three classical methods devised
by Pasteur for resolving inactive substances into their active
components. Mandelic acid may also be resolved by partial
hydrolysis of its esters by the ferment "lipase" (Dakin) ai.d
also by the partial esterification of the acid with an active alcohol
such as menthol (Marckwald).

Phenylmethylcarbinol.—The method of Grignard, of
which this preparation serves as an illustration, has received a
very wide application. The following is a brief and incomplete
list of these reactions, in which the organic radical (R) represents
within certain wide limits both an alkyl and aryl group :
Hydrocarbons. The magnesium compound is decomposed
by water,
RMgl + IIoO - R.H + Mgl(OH).
Alcohols may be obtained from aldehydes, ketones, esters,
acicl chlorides, and anhydrides,
R.                         1\      ,OMirI       Rx      ,OII
xco + RMo-i ->   xc;    " ->    >c<
R.                        fc            R      XR                 R'        R   .
^)           .                /OMgl       ^     /OMgl       ^   (    OH
v                              \R  "   J         \R                XR.
X  2